Devices for improving operating flexibility of steam-electric generating plants



Jan. 4, 1966 c. sTRoHMEYE-'JR 3,226,932

DEVICES Fon IMPRovING OPERATING FLEXIBILITY oF STEAM-ELECTRIC GENERATINGPLANTS Fllod June `'7. 1960 5 Sheets-Sheet 1 FLOW, lo OF RATED FLOW hisATTORNEY Jan. 4, 1966 c. sRoHMEYER, JR 3,226,932

DEVICES FOR IuPRovlNG OPERATING FLEXIBILITY 0F STEAM-ELECTRIC GENERATINGPLANTS Flled June 7, 1960 5 Sheets-Sheet 2 70o m .J I

z j soo LLI Q oQ H0035 so 25 20 l5 |o 5 o PRESSURE-HUNDREDS OF LBS/SCJN.INVENTOR Charles StrohmeyenJr. F|g.2.

his ATToR-E Y Jan. 4, 1966 c. STRQHMEYER, JR 3,226,932

DEVICES FOR IMPROVING OPERATING FLEXIBILITY OF STEAM-ELECTRIC GENERATINGPLANTS Flled June 7, 1960 5 SheetS-Sheet 3 F' -4 IRREHEAT TURBINEINLET|000 soo 3K1 xTRAcTioN 700 4mExTRAcT|oN Lu 600 f n: of lgjechon 50o|.P.REHEAT TuRBl T II (doynsr/eam of injeclo/n point) L 40o 0- E E 30oZOO lOO

O 2O 40 60 8O |00 |20 FLow, oF RATED FLow sTEAM F y 59, GENERATORINITIAL FTNAL STEAM g WATERWALLS a suPERHEATER n suPERHEATER GENERATORhis ATTORNEY 7* Jan. 4, 1966 c. STROHMEYER, JR 3,226,932 DEVICES FORIMPROVING OPERATING FLEXIBILITY OF Filed June 7. 1960 STEAM-ELECTRICGENERATING PLANTS 5 Sheets-Sheet 4 3 uz-'mp4s ii l I| [54 |l .1146

42l l l l 332 345 INJECTION STEAM REHEAT C ROSSOVER Fi nv vENToR 70Charles Strohmeyer, Jr. BY l I 58 (turbine exhaust) 40 his ATTORNEY Jan.4, 1966 c. sTRoHMEYER, JR 3,226,932 DEVICES FOR IMPROVING OPERATINGFLEXIBILITY OF STEAM-ELECTRI C GENERATING PLANTS 5 Sheets-Sheet 5 FiledJune '7. 1960 United States Patent This invention relates to devices andsystems for improving the operating flexibility of steam-electric lgenerating units including steam generators, turbine-generators andauxiliary equipments. s

Three basic problems that have given riseto consider- `able difficultyin the operation `of large units have been (l) the rather difficult and`prolonged start-up` time, (2) the inability to match steam and metaltemperatures in `the turbine during start-up and shut-down and (3) theinability to operate these units satisfactorily at lower loadcondition-s when this feature becomes desirable due to the` installationof future, more eiiicient generation.

An object of the invention is to provide a novel apparatus and systemproviding a complete `solution to the a-bovernentioned problems.

A more specific object of the invention is to provide a throttling valvelocated after the waterwall section and before the superheater steamoutlet of the steam generator.

A further specific object of the `invention is to provide a novel systemfor controll-ing or supervising steam conditions in the high pressureturbine during shut-down, start-up and low load operation to best suitturbine metal temperatures for both hot and col-d conditions. s

A still further specific object of the invention is to provide a novelsystem for controlling the steam temperature of the low pressure turbineexhaust during start-up or light load operation.

Other objects and advantages of the `invention will become more apparentfrom a study of the following description taken with the accompanyingdrawings wherein:

FIG. 1 is a typical load diversity chart in which thermal generation isplotted against time;

FIG. 2 shows steam enthalpy plotted against pressure, showingtemperature drop for a given pressure drop;

FIG. 3 shows steam temperatures in the high pressure turbine of aconventional reheat unit plotted against percent of rated ow.

FIG. 4 shows steam temperatures in the reheat intermediate and lowpressure turbines plotted against percent of rated flow. s

FIGS. 5a to 5h inclusive and FIG. 5j show various modifications ofthrottling valve systems located after the waterwall section and beforethe superheater steam outlet and between heat absorption conduits of thesteam generator;

FIG. 6 shows a system forcontrolling or supervising` Problem backgroundSteam-electric generating -plants presently in use have Patented Jan. 4,1966 volved in the following description. 'I'he same partially appliesto straight condensing designs.

It w-as originally planned that steam-electric generating units havingthe best heat rates would be operated continuously 24 hours a day athigh load factor. Therefore, shut-down and start-up economies were notgiven serious consideration at the time when these units were designed.The older less efficient units were to be operated at lminimum load orwere to be shut-down during t-he midnight hours or at other times whensystem demand was low, such as on weekends and holidays.

The ratio of high eliiciency conventional steam-electric generatingplants to total installed generating capacity is increasing, especiallysince an efficiency plateau based on economic factors has been reached.There are situations developing which require daily load reductions orshutdowns of high efiiciency steam-electric generating unit-s. Thisrequirement may be seasonal or on a continuing basis throughout theyear. Some of the responsible factors are: Y

(a) High ratio of high efficiency generating capacit to total generatingcapacity.

(b) Fuel cost differentials which favor generation in one area of autility system network vs. another.

`(c) Generation from an alternate source, as hydroelectric generation,where there is inadequate regulation of water flow which would requirewater to be wasted if steam generation were not curtailed; alternately,hydro generation costs during off peak hours may .be less than thermalgeneration fuel costs.

(d) Generation from future nuclear power plants. It is anticipated that,while the capital cost of the nuclear plant will ybe high, the fuel costwill be lower than in a conventional steam-electric generating plant.

(e) Increased number of shut-downs required for maintenance of highefficiency units.

A typical example of load diversity involving (a) and (c) above is shownin FIGURE 1.

`In changing load and in starting up or shutting down a steam-electricgenerating unit, one of the main considerations with respect to safeoperation relates to metal temperature changes in heavy walled equipmentcomponents. Excessive differential temperature across metal crosssections can cause stresses which exceed the elastic limit of thematerial and result in failures. Metal temperatures depend upon thesurrounding environment. If environmental temperature changes are toorapid or are unbalanced, the cross section differential temperatures maybe increased to points which produce permanent metal deformation. Wherethe environmental temperature conditions change with load, safeoperation indicates that load changes be made slowly, the rate beinginfluenced by the degree of change. On the other hand, if environmentaltemperature conditions could be maintained constant for all loads,temperature limitations with respect to load change would be eliminated.

Temperature problems must find their solution in steam generator andturbine designs which are thoroughly coordinated. There are limits forunit start-up depending upon the condition of the turbine and steamgenerator at time of start-up. One extreme is where a unit hascompletely cooled after a maintenance program or prolonged outage, theother extreme is where the unit is in a hot operating state after anunexpected trip-out.

Steam temperature characteristics for many steam generators are suchthat at rated steam pressure both superheat and reheat steamtemperatures droop with load in the lower range. To maintain metaltemperature differential within manufacturers tolerances, by control ofrate of cooling, it is necessary to shut the unit down slowly. Also,upon restart after the turbine has had time to cool still further, it isnecessary to proceed slowly to control the 3 rate of heating. Thischaracteristic is satisfactory when it is desired to overhaul a machineafter shut-down as the turbine can be cooled during the shut-down periodminimizing the waiting period before maintenance work can commence.

However, if the unit is to be removed from the line for power generationcost or load considerations only and restarted 6 to 8 hours later,prolonged shut-down and startup periods make this type of operationuneconomical. For a short shut-down of this type, the optimum conditionis Where the turbine can be shutdown rapidly in a hot state and duringrestart, steam can be supplied to the turbine at temperatures which willimmediately start restoring operating temperature levels at satisfactoryrates of rise without causing excessive temperature differentials amongcomponents.

It is impossible to accomplish the objective described in the precedingparagraph where steam generator outlet steam temperature at designpressure droops with load, unless some manipulation is employed.

In most turbines steam is admitted individually to separate chamberswhich supply the nozzles to the initial impulse stage. When steam is fedthrough the governing valves to each nozzle chamber sequentially andcumulatively, this is called partial admission. When steam is fedthrough the governing valve or valves to all the nozzles at the sametime and at the same rate, this is called full admission. Partialadmission takes maximum advantage of design throttle steam pressure atall loads and increases part load efficiency. For full admission,effective throttle steam pressure roughly decreases proportionally withload.

For very low steam flows to the turbine for either partial or fulladmission, the pressure upstream of the nozzles for the initial stage isvery low. Therefore, the turbine does not require design throttlepressure during low flow periods for satisfactory operation.

Pressure reduction of steam at constant enthalpy results in atemperature drop. Temperature drop for a given pressure drop isdependent upon the initial steam temperature before reduction ofpressure. These characteristics are shown in FIGURE 2. In reducing 2400p.s.i. steam at 1050 F. down to 200 p.s.i., there is a loss intemperature of approximately 120 F. In reducing 2400 p.s.i. steam at 750F. down to 200 p.s.i., there is a loss in temperature of approximately280 F.

If the pressure reduction of steam occurs in the superheating zone ofthesteam generator during low ow periods, the accompanying temperature dropincreases the temperature differential between the resultant fluid and/or vapor in the superheater tubes after pressure reduction and thehotter gas surrounding the tubes. This increases the transfer of heatfrom the gas to the fluid and/ or vapor and raises the heat content orenthalpy (B.t.u. per pound) of the superheater outlet steam Where thewaterwalls are operated at high pressures. For low loads, the turbinefirst stage nozzle chest and downstream steam pressures are a functionof flow and are substantially below design values. For any givenpressure, steam temperature is a function of enthalpy. Therefore, forlow iows, pressure reduction in the superheating zone of the steamgenerator above nozzle chest requirements raises steam enthalpy andtemperatures in the turbine first stage nozzle chamber and downstreamblade path. This is a desirable substitute for high pressure superheateroutlet steam during the hot condition where the boiler outlet steamtemperature characteristics are such that at design pressure,temperature droops with load. The pressure in the waterwalls may bemaximized, minimizing the time required to shut down the turbine hot andsubsequently to re-start a hot turbine.

If the above mentioned pressure reduction of steam occurs across theturbine steam supply admission valves, the accompanying temperature dropwill create a metal temperature differential in the valve body above andbelow the valve seat. For the hot start-up condition where the steamgenerator outlet steam temperature at design pressure decreases withload, pressure reduction across hot turbine steam supply admissionvalves can chill downstream metals and damage them. Such type ofthrottling should be minimized.

For starting a cold unit, rate of metal temperature rise should berestrained to avoid excessive metal stresses. Low superheater outletsteam pressures are desired because downstream metal temperaturesrapidly follow saturation steam temperatures as a result of steamcondensation. Saturation temperature is a function of pressure. Wheremetals are cold, raising steam pressure too rapidly will result in highheat transfer rates from the steam to the metal up to the saturationtemperature level. This,

'in turn, causes excessive temperature differentials in the metalcross-sections.

When the metal temperature is raised above the steam saturationtemperature for a given pressure, condensation will cease and heattransfer from the steam to the metal across a dry surface will greatlyreduce the heat transfer rate. The rate of metal heating may then becontrolled by regulation of steam flow quantity since heat transfer is afunction of both mass flow and temperature differential. For control ofrate of metal heating, steam pressure control when metal temperaturesare below saturation followed by flow control when metal temperaturesyare in the steam superheat range are more important than the amount ofsuperheat in the-steam. For starting a cold turbine, proper regulationof steam pressure and steam ow are essential.

A conventional drum steam generator may be operated at low pressures forcold start-ups. Once-through steam generators must be operated at fullpressure or close thereto inthe waterwall circuits to assure properdistribution of flow and density unless variable pressure operation Vis-built into the design at great expense.

Therefore, to economically satisfy the conditions in the paragraphabove, I have conceived the novel idea of reducing the pressure'afterythe waterwall'circuit and before the superheater outlet of the steamgenerator. This system may be used to improve turbine steam conditionsfor a cold or hot turbine when shutting down, starting up or operatingwith low load without dependence upon the pressure or temperature levelof the waterwalls for both drum and once-through steam generators. Thepressure reducing system has other advantages enumerated hereinafter.

FIG. 3 shows steam temperatures in the high pressure turbine of aconventional reheat unit plotted against percent of rated flow. From thesolid lines it can be seen that where throttle steam temperatureis heldconstant at 1050 F. and 2400 p.s.i.g. and employing partial admission,the first stage exhaust, first extraction and high pressure turbineexhaust steam temperatures decrease with load. The greatest temperaturechange occurs in the first stage which is the limiting condition withrespect to rate of load change.

The dash lines on FIG. 3 indicate high pressure turbine steamtemperatures where throttle steam temperature decreases with load;throttle pressure is held at 2400 p.s.i.g. and partial admission isemployed.

FIG. 4 shows steam temperatures in the reheat intermediate and loWpressure turbines plotted against percent of rated flow. Hot reheatsteam flow to the intermediate pressure turbine is of the full admissiontype. Pressure at any point inthe flow path from the inlet of the LP.turbine to the condenser is dependent upon the downstream ow. From thesolid lines on FIGURE 4 it can be seen that if the'reheat turbine inlettemperature is held constant, the downstream temperatures remainapproximately constant at all fiows except toward the exhaust end whichis affected the greatest extent by leaving losses. The exhausttemperatures would be excessive in the low flow range unless someprovision is made to correct this situation. This may be done byinjecting low enthalpy steam into the crossover between theintermediateand low pressure turbines.` The dot-dash lines showoperation with injection steam. This enables the reheat turbine to beoperated at near constant temperature over the entire load range. Thedash lines `show the effect of decreasing reheat inlet steam temperaturewith load.

Temperature changes in the reheat turbine need impose no restrictionswith respect to stop-start and load swing operation providing inletsteam temperatures are held near design values and cooling steam isinjected into the cross-over between the I.P. and LP. turbines whenthrottle admission flows `are small.

FIG. 8 shows the main steam and water cycle of the conventional reheatunit used for FIGS. 3 and 4 and is typical of the steam-electricgenerating unit described in this specification. The solid lines arewater conduits and the dash lines are steam conduits. All components arestandard and are produced commercially. Flow control valves and bypassesin the various conduits are omitted for the purpose of simplification.They would be arranged in a conventional manner and are not a part ofthis invention. Direction of ow is indicated by the arrows in the steamand water conduits. The high pressure and intermediate pressure reheatturbines are connected by a common driving shaft which drives anelectric generator. The low pressure turbines also drive their ownelectric generator. Extraction steam ows are from intermediate stagesbetween turbine blade rows in the turbine steam iiow path. FIG. 8 showsthe general relationships of the plant components. It will be understoodthat details as the arrangement of the turbine elements, extractionpoints, feedwater heaters, pumps and other auxiliaries can vary toaccomplish the same overall results within the intent of the cycleillustrated in FIG. 8. Also, there may be more than one stage of steamreheating in the steam generator associated with the cycle as is knownto exist in commercially operating plants.

General description of present invention Generally stated, there arethree separate but interrelated control elements involved in the presentinvention, namely:

Item 1.--A throttling and shut-off valve system located after thewaterwall section and before the superheater steam outlet of the steamgenerator and between heat absorption conduits, such as shown in'FIGS.5a and 5b. Various modifications of such System are shown schematicallyin FIGS. 5a to 5j inclusive. This system per mits the steam generator tobe operated at two pressure levels, the lowest level being downstream ofthe valve. This system may stand on its own merits and does not requirethe systems described in items 2 and 3 below.

Item 2.-An automatic control or a supervisory system governing start-upand shut-down of the steam and turbine generator. Such system is shownin FIG. 6. Superheater outlet steam enthalpy is controlled from steamtemperature in the turbine first stageexhaust or subsequent downstreampoint before reheat is added to the steam. Turbine throttle pressure iscontrolled from the differential between steam and metal temperature inthe throttle valve or governor valve chest, or is controlled to aconstant pressure, or to a pressure variation programmed with time, loador flow, or is controlled to increase enthalpy of the steam generatorsuperheater outlet steam during low steam flow periods. Temperaturedifferentials between the turbine nozzle chests and/ or rate of metaltemperature change of parts in or a part of the high pressure turbinecylinder, which are exposed to primary steam supply to the first stagenozzles, limit the rate of throttle steam enthalpy rise and rate ofturbine load increase.

Item 3.--A system for controlling the steam temperature of the lowpressure turbine exhaust during start-up or light load operation.` Suchsystem is shown in FIG.

7. Steam from the steam generator drum, low temperature superheater,high temperature superheater after attemperation or desuperheating, orfrom the steam generator starting bypass system, is injected into thecrossover between the low pressure turbine and the upstream turbineelement. The increased mass flow through the exhaust decreases leavinglosses per pound of steam flow and lowers exhaust steam temperature. Thelower enthalpy of the injection steam reduces the temperature of themixture in the crossover before entering the low pressure turbine. Lowpressure turbine exhaust temperature is controlled manually orautomatically by a temperature detector or detectors located in the lowpressure turbine exhaust steam flow or in the exhaust structure metal.This (or these) in turn control/s injection steam flow. The amount ofinjection steam flow during low loads may also be controlled at aconstant or variable rate by a signal fed to the injection steam supplyvalve controller from generator load, turbine governor system, turbinestage pressure or ow.

Specific description of present invention FIGS. 5a and 5b shows twodifferent locations for the throttling valve T in the solid line fluidconduits between the steam generator waterwalls and superheater outletand between heat absorption conduits indicated by the saw tooth solidline. The steam generator fluid circuits include a steam drum or steamand water separator D for pressures below critical. Such drum orseparator D may `be omitted from the circuit for once-through steamgenerators designed for subcritical or supercritical pressures. Athrottling and shut-off valve T is located in each of one 0r moreparallel ow circuits as shown in either FIG. 5a or 5b. The arrowindicates the direction of fiow in the fluid conduits.

While not specifically shown on FIGS. 5a and 5b, a conventional drumtype steam generator has internal recirculation conduits connecting thesteam drum and the steam generating waterwalls.

Part of the fluid discharge from the steam generator waterwalls may bedrawn off from the main flow circuit before throttling valve T throughbypass line BP. Bypass BP may return to the cycle wholly or in part atsome point 4between the superheater outlet and the water supply point tothe steam generator. Bypass BP may discharge fluid from the drum orsteam and 'water separator D, or may discharge fluid from any otherdownstream point in the main flow line before throttling valve T, in thecase where the drum or steam and water separator D is included in themain circuit. Bypass BP may discharge fluid from any point in the mainow line downstream of the `waterwalls vup to throttling valve T in thecase where the drum or steam and water separator is not included in themain circuit.

The throttling valve in one single flow line or in multiple parallelflow lines may consist of a single valve in each ow line.

FIG. 5c shows a multiple or parallel flow line consisting of a singlemain line valve V or valves in the solid line uid conduit connected tothe arrow and one or more smaller bypass valves in the dash line fluidconduit connected `to the solid line fluid conduit.

FIG. 5d shows a throttling valve V in a single ow line or in multipleparallel flow lines consisting of a valve in all parallel circuits toeach end of a main ow line, the parallel circuits being shown as solidand dash lines.

Any one or all of the valves of the various throttling systems describedmay be manually or power operated. The power operated valves may becontrolled by any `one or a combination of the following systems usingconventional and known components.

FIG. 5e shows a power operator P for the throttling valve V governed byan operator controller C through the connecting dash line controlcircuit. The valve operator controller C is manually set to open -orclose the throttling valve to any end or intermediate position.

FIG. 5f shows a system in which the throttling valve V is opened orclosed by pressure controller PC and the connecting dash line controlcircuit to control downstream fluid pressure in the solid line fluidconduit to any preset constant pressure or programmed pressure variationwith time, ow or generator load which is lower than upstream pressure.Pressure controller PC receives its pressure impulse from pressure tapPT located in the downstream steam pipe and the connecting dash linecontrol circuit.

FIG. g shows a system in which the throttling valve V in the seriallyconnected solid line fluid conduit and between the steam generatorwaterwalls and superheater outlet and upstream of superheating heatabsorption conduits (indicated by the saw-tooth solid line) is opened orclosed to control downstream uid pressure in the said serially connectedfluid conduit which, in turn, controls steam temperature in the saidserially connected fluid conduit downstream of the superheater elementand in an intermediate portion of the turbine as shown in FIG- URE 8between turbine stages S. Steam temperature measurement TM through theconnecting dash line control circuit to temperature controller TCthrough the connecting dash line control circuit to C actuates thethrottling valve V upstream of all Or a portion of the superheater.

FIG. 5h shows a system where two or more power operated valves V in thesolid line uid conduits are included in the throttling system, they maybe operated in parallel or sequentially by controller SC through theconnecting dash line control circuits to control downstream fluidpressure or temperature as described above.

FIG. 5j shows a system wherein the throttling valve V located in thesame manner as in FIG. 5g above is opened or closed to controldownstream fluid pressure in the said serially connected solid linefluid conduit which, in turn, controls downstream steam temperature inthe said serially connected fluid conduit to limit the temperaturedifferential between 1) the steam temperature in the said seriallyconnected fluid conduit at some point downstream of the superheateroutlet and (2) the uid conduit metal temperature. For a given steamenthalpy, raising pressure after the throttling valve raises the steamtemperature, and lowering pressure after the throttling valve lowers thesteam temperature. When the steam temperature is higher than the conduitmetal temperature, heat will ow from the steam to the metal. As themetal rises in temperature, the differential temperature will diminish.The differential temperature controller DC receives steam and metaltemperature measurements from points TM through the connecting dash linecontrol conduits and will cause the throttling valve to open through theconnecting dash line control conduits to C raising steam pressure andtemperature. In this manner, rate of metal temperature rise can becontrolled. When the steam temperature is below metal temperature, heatwill flow from the metal to the steam. Lowering steam pressures by meansof the differential controller DC controls rate of metal cooling. Rateof heating or cooling can be adjusted by the degree of temperaturedifferential.

As an alternate or supplement to the above paragraph, the throttlingvalve can control the steam temperature at some point in the saidserially connected fluid conduit downstream of the superheater outlet toany preset constant temperature, or programmed temperature variationwith time, llow, generator load or turbine stage pressure without regardto the differential temperature between the steam and metaltemperatures. In such case only steam temperature measurement would berequired in FIG. 5]'. Differential controller DC would become thetemperature controller TC.

The throttling valve can supply low pressure steam to the turbine. Thepressure in the steam generator waterwalls and evaporating circuits canbe maintained at higher pressure and saturation temperature. Thisaccelerates the rate at which load from the steam generator can beincreased during hot starts. Steam throttling can be divided between thepressure reducing system in the boiler and the turbne steam admissionsystem, minimizing quenching in the turbine steam admission system for ahot start and saturation pressure and temperature differential acrossthe turbine throttling device for a cold start. In starting up a unit,to control steam temperatures in the turbine, dual pressure operation ofthe steam generator (higher pressure upstream and lower pressuredownstream yof the throttling) permits changes in tiring rates whichincrease or decrease upstream steam pressure as well as superheateroutlet steam enthalpy at a predetermined downstream pressure. l

Pressure reduction of steam before the superheater outlet permitsincrease in the energy level of the outlet steam for the :sametemperature. Enthalpy of steam at 2400 p.s.i.a. 1050 F.=l494.2B.t.u./lb., enthalpy of steam at 1000 p.s.i.a. 1050 F.=l533.2 Btu/lb.This can be aC- complished while maintaining saturation temperatureupstream of pressure reduction at a levelhigher than that correspondingto the superheater outletvsteam pressure. Saturated temperature at 2400p.s.i.a.=662.12 F., and at 1000 p.s.i.a.='556.3 l F. This permits thesteam generator to produce a high enthalpy, low pressure steam supplyfor starting a hot turbine without degrading tempera- ,ture level of thewaterwalls by operating the entire steam generator at a lower pressure.

The throttling valve permits the upsteam waterwalls to be `operated at a'higher pressure than the downstream superheater. Reducing the pressurebefore the final superheater lowers the steam temperature below what itwould have been at a higher pressure without the pressure reducingsystem. This increases the temperature gradient between (l) the gas and(2) the resultant liquid and/or steam after pressure reduction. Thisresults in greater heat transfer which increases the enthalpy of thesuperheater outlet steam. The above control can be used to advantage tocontrol cooling or heating in a turbine during shut-down or start-upoperations.

The bypass BP may be used to establish the necessary flow throughonce-through steam generator'waterwalls when firing the furnace prior toestablishing substantial flow through the throttling and shut-off valvesystem. The bypass BP may be used to increase firing rate to increasesuperheater outlet steam enthalpy.

The steam generator heat absorption conduits shown on FIGS. 5a and 5billustrate a steam generator having interdependent components betweenthe steam generator waterwall inlet and superheater steam outlet whensteam is discharged through the superheater outlet conduit t-o a steamconsumer as the turbine shown in FIG. 8. `There is no means forbypassing flow around heat absorption components for taking them out ofservice when the combined unit is in normal operation after startup. Thebypass BP as described above may be used for starting up a once-throughsteam generator or increasing superheater outlet steam enthalpy.

The objective of the control system embodying my in- Vention shown inFIG. 6, is to provide a safer steam-electric generating unit whenstarting up and shutting down by restricting metal temperature rate ofchange and controlling steam temperatures to eliminate damage fromthermal stress. Such a control system can provide the operator withindication of a comparative and supervisory character or it canautomatically control part or all of the related boiler and turbineplant equipment.

The high pressure turbine 44 rst stage exhaust steam temperature at 43,or steam temperature at another downstream location 2 before reheat isadded to the steam, controls the enthalpy of the steam to the turbine44. The steam temperature in the turbine steam flow conduits 43 ismeasured by temperature detector l. 2 is an alternate location fortemperature detector 1 and would be connected at 2 in the same manner asis shown for temperature detector 1. All of the control components shownare conventional and known. The signal from detector trol point.

9 1 is fed through con'trol conduit 42 to` steam temperature recorderand/ or controller 3 which controls steam temperature at 1 to a presetadjustable value which best suits the temperature condition of theturbine 44 during start-up or shut-down.

When steam temperature deviates from the control point, a signal fromcontroller 3 is sent through control conduit 45 to 4 to restoretemperature at 1 to the control point. 4 may be one or more -of thefollowing: steam generator internal flow bypass valve operator controls,fuel feed controls, gas bypass damper controls, excess air or gasrecirculation or gas tempering controls, attemperation or desuperheatingspray water controls, or a device to bias heat absorption ratio betweenevaporating and superheating duty, to suit the specific boiler designinvolved. The temperature detector 5 measures superheater outlet steamtemperature in the serially connected fluid conduit 9. The signal fromdetector 5 feeds to controller 3 through control conduit 47 and may beused to record, alarm or limit the control action of controller 3 and isoptional. The temperature detector 6 measures metal temperature adjacentto detector 1. The signal from detector 6 feeds to controller 3 throughcontrol conduit 46 and may be used to record or alarm differentialbetween detectors 1 and 6 above preset values. Detectors 6 is optional.k7 is an alternate location for 6 associated with location 2 above.Detector 6 would be connected in the same manner as shown if installedat location 7. Where supervisory indication only, is required, recorder-or controller 3 just indicates temperatures and/or indicatestemperature dilferentials and/or actuates an alarm. Control conduit 45would be eliminated.

Rotating blades 49 are mounted on turbine spindle 50.`

Stationary blades 51 are mounted on the turbine stationary frame 52.Steam ilow through conduits 43 passes through the blades 49 and 51performing work and driving the turbine shaft 53.

Steam temperature on the above seat side of the governor valve(s) 54 insteam chest 17 in connecting conduit 8 (throttle/ stop valve(s) 18 open)is controlled above or below metal temperature at 11 to regulate therate of governor valve chest 17 heating or cooling, respectively. Sincesteam enthalpy is controlled at point 1, steam temperature in thegovernor valve chest 17 may be varied the metal temperature at 11, heatwill ow from the steam to the metal. As the metal rises in temperature,the differ-` ential temperature Will diminish. Differential controller12 will feed a signal through control conduit 55 into operatorcontroller 13 and through control conduit 56 which actuates the poweroperator 57 on throttling valve(s) 14 which may be located betweenthelsteam generator Waterwalls and superheater outlet and between heatabsorption conduits. Throttling valve(s) 14 open raising steam pressureand temperature. Thus, the rate of metal temperature change can becontrolled by adjusting the temperature differential control setting ofcontroller 12. When the steam temperature is below metal temperature,heat will flow from the metal to the steam. The rate of metal coolingcan be controlled by lowering the steam pressure with controller 12. t

Temperature detector 15 is equivalent to detector 10 and would beconnected to 12 through conduit 48 and detector 16 is equivalent todetector 11 and would be connected to 12 through conduit 54 when thethrottle/ stop valve/s 18 is/ are used as an alternate or substitutecon` Power operator 58 actuates the governor valves 54 and t l0 poweroperator 59 actuates the throttle stop valve/s 18'. Power operators 58and 59 are associated with the turbine 44 speed controls (not shown).

The boiler is normally operated with constant superheater outletpressure. When throttling system 14 is used to control steam pressuresat 8 or 9, the pressure upstream of 14 can be controlled by steampressure controller 19. Pressure measurement means for such case isshown at 20. Control conduit 60 connects 19 and 20. Where supervisoryindication only is required, 12 just indicates temperatures and/ orindicates temperature differentials and/ or actuates an alarm. Where thethrottling valve/s 14 is/are located before the superheater outletbetween heat absorption conduits, temperature detector 21 throughcontrol conduit 61 to 3 functions in the same manner as 5, to preventthe temperature at 21 from exceeding a preset limit.

As an alternate to the above, pressure at points 8 or 9 can becontrolled by a pressure tap as a substitute for detector 10 or 15,which tap feeds an impulse to a pressure controller as a substitute forcontroller 12. Detectors 11 and 16 and conduit 54 are not required. Thepressure controller 12 controls steam pressure at 10 or 15 to any presetconstant pressure or programmed pressure variation with time, or load,or flow, or boiler outlet steam' high enthalpy capability. Pressurecontroller 12 feeds signals to controller 13 through conduit 55 and from13 to 57 through conduit 56 to actuate valve 14.

In starting up a unit where the steam-generator evaporating circuitpressure has decayed during shut-down,

pressure controller 19 may be non-operative while pressure is beingraised to design level after firing is commenced. Pressure in theevaporating circuits may be allowed to float below design level aspermitted by the steam generator design and water conditions so thatincreased or decreased firing rate will raise or lower pressure upstreamof throttling valve/ s 14as Well as increase or decrease steam enthalpyas required by controller 3 where the firing rate is used as a devicefor controlling the turbine first sta-ge exhaust steam temperature.Normally, after start-up, the pressure in a conventional drum type steamgenerator evaporating circuits will drift up to design level. In doingthis, small pressure drops in the evaporating circuits, as a result oftemporary decreased tiring rates, provide quick response for correctinghigh supert heater outlet steam enthalpy.. The corrections can be madeindependently of pressure control downstream of valve/s 14.

The following system is optional regarding the overall control system.As a result of the above described systems, metal temperature changesupstream of steam throttlng to `the initial stages of the turbine anddownstream from the first stage during start-up and shut-down can belimited through controls. After 'switching from throttle/stop valve/ssteam admission control (full admission) to governor valve control(partial admission), steam temperature at 43 after the rst stage willdroop. As steam enthalpy increases to raise temperatures after the irststage, nozzle chest/s temperature/s will Ie. The following control isintended to limit nozzle chest temperature rise beyond acceptablelimits. Temperature detector 22 located in the No. 1 governor valvenozzle chest 62 o-r other internal portion of the high pressure turbinesends a signal to temperature recorder and/or controller 23 throughcontrol conduit 63. The controller 23 operates from rate of temperaturechange function. I f the rate of` change exceeds a preset value,controller 2 3 sends a si-gnal to steam temperature controller 3 throughcontrol conduit 64 counterbalancing .the signal resulting fromtemperature change at 1. 26 is an alternate loca.- ton for 22 and wouldbe connected to 23 through conduit 63.

As an alternate or supplemental control for the system described inthepreceding paragraph, there are two or more temperature detectors 22. Oneis located in the metal of the No. 1 governor valve nozzle chest,attached or equivalent part and the othe-r/s is/ are located in themetal of another/other governor valve nozzle chest/s or other part/ sassociated with steam admission to the first stage. The impulse fromeach of detectors 22 is fed through control conduits 63 to a temperaturedifferential recorder and/ or controller as an alternate for orsupplement to 23. If the temperature differential between any twoassociated points exceeds a preset valve, controller 23 sends a signalthrough control conduit 64 to steam temperature controller 3counterbalancin-g the signal resulting from temperature change at 1.Where supervisory indication only is required 23 just indicate-stemperature and/or indicates temperature differentials and/or actuatesan alarm control conduit 64 is not required.

Controller 23 can be used to control rate of load change. In such case asignal is sent from controller 23 to controller 41 which increases,holds constant or decreases the turbine speed changer or governorsetting (not shown). For start-up, increase above controller 23 setpoint decreases or holds constant turbine steam ilow; decrease belowcontroller 23 set point increases turbine steam tlow. Reverse forshut-down. Controller 23 signal may merely limit another preset rate ofload change (not shown).

The following system is also optional. Temperatureload change comparator25 is equipped with a chart (not shown) having a relatively fast speedsuch as one inch/ three minutes. Temperature at 22 through conduit 63 tocontroller 23 through control conduit 65 to comparator 25 and generatorelectrical output from 24 through control conduiti66 to comparator 25are separately registered on the chart in comparator 25. The slope ofthe two plots and, their juxtaposition one with the other indicateacceptable load change with respect to rate of metal temperature change.C

The control systems described above and shown v1n FIG. 6 are convenientones which can be made automatic or used for supervisory purposes at atime when there are numerous operations taking place in the plant. Theoperators duties are simplified during start-up or shutdown.

FIG. 7 as described hereinabove shows a system for controlling the steamtemperature of the low pressure turbine exhaust during start-up or lightload operation. Steam from the boiler drum, low temperature superheater,high temperature superhcater after attemperation or desuperheating, orfrom the steam generator starting bypass system, is injected into thecrossover to the low pressure turbine from the upstream turbine element.Where high pressure turbine exhaust steam is maintained close to designvalues for hot starts, the turbine reheat inlet steam temperature willalso be increased. In order to prevent excessive temperature rise in thelow pressure turbine exhaust, I devised the system shown in FIGURE 7. Itis possible to inject spray water in the exhaust hood as an alternate.Since the spray water enters a Ihigh velocity steam zone, many peopleare afraid of erosion` from this type of cooling. It is necessary tocool the exhaust hood to prevent excessive expansion in the upwarddirection which would cause the low pressure turbine bearings to riseand take excessive shaft loading. This would disrupt shaft alignment andcause shaft eccentricity along with vibration. The injection steam tothe crossover contains superheat and eliminates dangers `associated withimproper mixing of spray water. Also steam injection for coolingpurposes into the crossover between the intermediate and low pressureturbines provides proper temperature distribution between the turbineelements. The last stage steam temperature is maintained at the exhausthood temperature level.

FIG. 7a shows portions of the physical structures illustrated in FIG. 7.FIGS. 7b and 7c are enlarged views of FIG. 7a. Steam from a highpressure turbine and steam generator reheater (not shown) ilows throughan intermediate pressure reheat turbine 31 thence through crossover pipe30 which is connected to a low pressure turbine 32. Injection steam fromthe steam generator drum, intermediate or iinal superheater, or from theboiler starting up bypass system, is supplied to crossover pipe 30through pipe 33 and distribution chamber 35. Holes 36 admit steamuniformly from distribution chamber 35 t0 crossover pipe 30. Injectionsteam ow to crossover pipe 30 is regulated by a power operated controlvalve 34, such as an air operated valve. A pair of air or oil operatedvalves 37 protect the turbine from overspeeding by closing when theturbine trips. Control valve 34 is actuated by controller 38 throughcontrol conduit 67, controller 38 receiving temperature detectionsignals from measuring element 39 located in the exhaust steam flow pathor from measuring element 40 located in the metal of the exhauststructure. Control conduit 67 connects 38 and 39. Control conduit 68connects 38 and 40. As a supplement or alternate, controller 38 may bemanually set to control the opening of valve 34 to any preset fixedposition or be set to automatically control the valve 34 position fromcontroller 69 and connecting control conduit 70 programmed with time,ow, generator load or turbine stage pressure independent of temperaturedetectors 39 and 40 and control conduits 67 and 68. If the additivesteam supply exceeds an enthalpy of 1200 B.t.u./lb. it should bedesuperheated beforehand. The best enthalpy value ranges between 1150and 1200 B.t.u./lb.

Thusit will be seen that I have provided ethcient devices and systemsyfor improving the operating flexibility of steamelectric generatingunits, including a throttling valve located after the waterwall sectionand before the superheater steam outlet and between heat absorptionconduits of the steam generator to enable the steam generator to beoperated at two pressure levels, the lowest level being downstream ofthe valve system; also including an automatic control or a supervisorysystem governing start-up and shut-down; also including a system forcontrolling the steam-temperature of a low pressure turbine exhaustduring start-up by injecting controlled amounts of steam into thecrossover between the low pressure turbine and the upstream turbineelement; also including other systems for improving operation andelliciency of steam-electric generating units.

While I have illustrated and described several embodiments of myinvention, it will be understood that these are by way of illustrationonly, and that various changes and modifications may be made within thecontemplation of my invention and within the scope of the followingclaims.

I claim:

1. A steam-electric generating plant comprising a steam generator havingheat absorption conduits including waterwalls and a superheater, a steamturbine including steam admission controls, fluid conduit means seriallyinterconnecting said waterwalls, superheater and steam turbine,throttling valve means located in said serially connected iluid conduitmeans between said waterwalls and at least a portion of said Superheaterand which is operable from the fully open to the fully closed workingposition, and control means for effectively regulating the steamtemperature in a portion of said turbine for varying load conditions,said control means being responsive to a variable condition of saidturbine for selectively opening said throttling valve means so as toselectively vary the pressure in said superheater portion in a rangebetween the pressure which results from the valve means when in thefully open working position, down to the minimum throttled positionrequired to admit only sufcient stea-nrto said turbine to satisfy anyrequired turbine load.

2. A high pressure steam-electric generating plant comprising .a steamgenerator having heat absorption conduits including waterwalls and aSuperheater, a steam 13 turbine drive having a high pressure turbine andineluding steam admission controls at the inlet to the high prssureturbine, fluid conduit means for` serially connecting together saidwaterwalls, superheater and ste-am turbine drive, throttlig Valve meanslocated in said serially Connected Huid conduit means beyond saidwaterwalls and upstream of at least a portion of said superheater, selective opening of said throttling valve means producing variabledownstream fluid pressure, control means for regulating iluid pressureupstream of said throttling valve means, and for controlling saidthrottling valve means and said steam admission controls and beingresponsive to a variable condition of said turbine drive for controllingsaid'variable downstream iiuid pressure independently of the upstreamfluid pressure in said waterwalls, so as to selectively raise, lower andstabilize steam temperature within said steam turbine drive duringshutdown, startup and low load operation by control of the said variabledownstream pressure down to the limit required by said steam admissioncontrols to pass suiiicient steam to maintain operating load of saidsteam turbine drive.

3. A high pressure steam-electric generating plant as recited in claim 2together with a water inlet to said waterwalls, conduit means having oneend connected to said serially connected iluid conduit means downstreamof said Waterwalls and before said throttling valve means, and the otherend being connected to said water inlet for circulating the necessaryflow in said waterwalls prior to establishing substantial llow throughsaid throttling valve means.

4. A high pressure steam-electric generating plant as recited in claim2, including an intermediate pressure turbine and a low pressure turbinein conjunction with said high pressure turbine, a reheater incorporatedin said steam generator, said serially connected fluid conduit meanscontinuing through said high pressure turbine to and through saidreheater to and throughsaid intermediate pressure turbine to and throughsaid low pressure turbine, and means for controlling the iluidtemperature of said low pressure turbine exhaust during low loadoperation comprising means responsive to said exhaust temperature forinjecting iluidinto said serially connected uid conduit means betweensaid intermediate pressure turbine and said low pressure turbine atlower temperature than the uid exiting from said intermediate pressureturbine, thereby lowering iluid temperature and increasing fluidquantity to said low pressure turbine.

5. In a steam-electric generating plant comprising a steam generatorhaving heat absorption conduits includ- ,ing waterwalls .and asuperheater, a steam turbine including steam admission Valve means,iluid conducting conduit means serially interconnecting said waterwalls,superheater and steam turbine, throttling valve means located in saidserially connected conduit means between said waterwalls and at least aportion of said superheater, the method of controlling the metaltemperature of a portion of said steam turbine structure which comprisesselectively closing said throttling valve means from its fully openworking position to a substantially throttled position While openingsaid steam admission valve means so as to selectively decrease the duidpressure lin the said superheated portion, whereby the metal temperatureof said steam turbine portion is selectively varied to satisfy differentload conditions. n

References Cited by the Examiner UNITED STATES PATENTS 1,767,714 6/ 1930Stender 122-460 1,832,150 11/1931 Stender 122-448 1,942,861 l/ 1934Huster 122-1 2,346,179 4/1944 Meyer et al. 60-104 X 2,590,712 3/ 1952Lacerenza 122-479 2,602,433 7/ 1952 Kuppenheimer 122-479 2,649,079 8/1953 Van Brunt 122-479 2,811,837 11/1957 Eggenberger 60-73 2,918,798'12/ 1959 Schroder 60-7-3 2,989,038 6/1961 Schwarz 122-406 3,009,32511/1961 Pirsh 60-105 3,019,774 2/ 1962 Beyerlein 122-406 3,035,5575/1962 Litwinoft 122-479 FOREIGN PATENTS 946,148 7/1956 Germany.

201,304 8/ 1923 Great Britain.

236,253 7/ 1925 Great Britain.

234,093 8/1926 Great Britain.

851,784 10/11960 Great Britain.

879,032 10/ 1961 Great Britain.

116,651 9/ 1926 Switzerland.

OTHER REFERENCES Combustion, December 1956; page 46. German applicationNo. 1,043,347; printed November Mitteilungen: German publication issueof September 1956.

SAMUEL LEVINE, Primary Examiner:

ABRAM BLUM, ROBERT R. BUNEVICH,

Examiners.

1. A STEAM-ELECTRIC GENERATING PLANT COMPRISING A STEAM GENERATOR HAVINGHEAD ABSORPTION CONDUITS INCLUDING WATERWALLS AND A SUPERHEATER, A STEAMTURBINE INCLUDING STEAM ADMISSION CONTROLS, FLUID CONDUIT MEANS SERIALLYINTERCONNECTING SAID WATERWALLS, SUPERHEATER AND STEAM TURBINE,THROTTLING VALVE MEANS LOCATED IN SAID SERIALLY CONNECTED FLUID CONDUITMEANS BETWEEN SAID WATERWALLS AND AT LEAST A PORTION OF SAID SUPERHEATERAND WHICH IS OPERABLE FROM THE FULLY OPEN TO THE FULLY CLOSED WORKINGPOSITION, AND CONTROL MEANS FOR EFFECTIVELY REGULATING THE STEAMTTEMPERATURE IN A PORTION OF SAID TURBINE FOR VARYING LOAD CONDITIONS,SAID CONTROL MEANS BEING RESPONSIVE TO A VARIABLE CONDITION OF SAIDTURBINE FOR SELECTIVELY OPENING SAID THROTTLING VALVE MEANS SO AS TOSELECTIVELY VARY THE PRESSURE IN SAID SUPERHEATER PORTION IN A RANGEBETWEEN THE PRESSURE WHICH RESULTS FROM THE VALVE MEANS WHEN IN THEFULLY OPEN WORKING POSITION, DOWN TO THE MINIMUM THROTTLED POSITIONREQUIRED TTO ADMIT ONLY SUFFICIENT STEAM TO SAID TURBINE TO SATISFY ANYREQUIRED TURBINE LOAD.